This invention relates to an improved noncatalytic process for reducing the concentration of nitric oxide (NO) from combustion effluents by injecting ammonia into a combustion effluent at elevated temperatures. In particular, this invention relates to a process for minimizing ammonia breakthrough in such a process while keeping the formation of additional NO at a minimum.
Various oxides of nitrogen, particularly nitrogen dioxide, are known to cause deleterious effects in human, plant, and other animal life. Moreover, certain of these oxides have been identified as reactants in photochemical smog formation. Consequently, the emission of these nitrogen oxides to the atmosphere poses a threat to the health and welfare of the community. As a result, considerable effort is being made to minimize the emission of these oxides to the atmosphere. A major source of these nitrogen oxides is NO contained in combustion effluents emanating from such stationary sources as gas and oil-fired steam boilers for electric power plants, process furnaces, municipal incinerators, and coal-fired utility boilers.
Considerable efforts have been made in the art to develop methods for removing NO from combustion effluents. U.S. Pat. No. 3,900,554, which is incorporated herein by reference, describes a variety of processes, both catalytic and non-catalytic, for reducing the concentration of NO in combustion effluents. It is recognized in the art that the non-catalytic processes, such as the process claimed in U.S. Pat. No. 3,900,554, are more desirable from an economic point of view provided the desired degree of NO reduction can be achieved at all furnace or boiler loadings. Catalytic processes require the use of an expensive catalyst which may be poisoned by various impurities, such as sulfur oxide, steam, soot, and the like, in the combustion effluent.
Although several of these prior art processes have been proven successful in varying degrees for reducing NO in combustion effluents, there still remains a problem of ammonia breakthrough especially when excess ammonia is employed to further reduce NO emission. That is, some of the ammonia may remain unreacted and be emitted in the combustion effluent as a pollutant, thereby partially defeating its purpose as an agent for removing NO.